The present invention relates to the field of implantable medical devices and more particularly to chronically implantable medical devices provided with a porous, self-assembling, cross-linking mono or multi-molecular film.
It has become common to provide therapy and treat diseases using implantable medical devices (IMD) that are chronically implanted within the body of a patient. Examples of such medical devices include pacemakers, defibrillators, drug-delivery devices, and electro-stimulators for stimulating nerves, muscles, and other tissue.
One problem associated with the chronic implantation of IMDs involves the growth of fibrous tissue around the device. When a foreign object such as an IMD is introduced into a patient's body, the body's auto-immune system forms a collagen capsule around the foreign object. This capsule, which has fibrous tissue, attaches to the IMD in a manner that prevents easy extraction of the device. This makes it difficult to replace or re-locate a medical device after it has been in the body for any significant amount of time. This problem is particularly prevalent when dealing with implantable medical leads.
Implantable medical leads have many uses. For example, leads carrying electrodes and other sensors are often positioned within a chamber of the heart or in the associated vasculature. These leads may be used to deliver electrical stimulation to cardiac tissue, and/or to sense and detect physiological signals. Leads may also be utilized to deliver medication to the body as controlled by a drug delivery device. Leads may also release biologic agents or carry diagnostic and monitoring tools into a tissue or an organ.
As noted above, the formation of fibrous tissue surrounding an implantable medical lead results in problems when the lead is to be replaced or re-located. The problems are exacerbated by the formation of small micro cracks in the surface of the electrode body. These cracks result when leukocytes release oxygen-free radicals causing an autoxidation reaction at the electrode's surface. The small crevices create additional surface area and spaces within which fibrous tissue can bond, making chronic lead extraction even more difficult.
Many methods have been devised in attempts to prevent the bonding of collagenous capsule tissue to the surface of IMDs. If such bonding could be prevented, the extraction of chronically-implanted devices would be greatly simplified. One manner of attempting to prevent tissue in-growth describes coating a lead with a porous polytetrafluoroethylene (PTFE) layer having a pore size of less than 10 microns or smaller so that tissue in-growth is prevented.
Other methods of preventing tissue in-growth are directed more specifically at eliminating the formation of tissue around the electrode structures carried on some lead bodies. One technique involves injecting silicone rubber into the spaces between the individual coils of an electrode structure. The resulting thin coating of silicone rubber surrounding the exterior of the electrode coils minimizes tissue in-growth between the filars of the coils, while leaving a portion of the coils exposed to deliver electrical stimulation to a patient.
Although the foregoing mechanisms have been developed in attempt to prevent collagen formation with the surface of an IMD, problems still remain. Therefore, what is needed is an improved device and method to prevent tissue in-growth on the surface of a chronically-implanted medical device.